: The long-term objective of this research is to better understand certain individual and group differences in audition. Many auditory measures exhibit large sex differences. Among these are otoacoustic emissions (OAEs) and auditory evoked potentials (AEPs). OAEs are sounds produced by the cochlea, and AEPs are gross potentials detectable with scalp electrodes following the presentation of a click stimulus. Because sex differences exist in the OAEs and AEPs of infants and children as well as in adults, they must originate, at least in part, during prenatal development. Because many other sex differences in body, brain, and behavior are known to be produced by differential exposure to androgens during prenatal development, the implication is that the sex differences in OAEs and AEPs also are produced by differential androgen exposure. To test this implication, OAEs will be measured in two colonies of rhesus monkeys that were treated with androgenic or anti-androgenic agents during prenatal development. If prenatal androgen exposure is a controlling factor, then those monkeys exposed to higher than normal levels of androgens prenatally should have fewer and weaker OAEs than monkeys exposed to lower levels of androgens. In past research, two groups of human females were found to have atypical OAEs. Monozygotic (MZ) female twins had nearly twice the number of OAEs as same-sex dizygotic (SSDZ) female twins or female nontwins, and females having a male co-twin (opposite-sex dizygotics or OSDZs) had fewer and weaker OAEs than SSDZ or nontwin females. In one proposed experiment, the AEPs of MZ and OSDZ females will be measured to see if their auditory brains are atypical as well as their cochleas. In another series of experiments, various behavioral (psychoacoustical) measures will be made on MZ and OSDZ females to determine if their atypical OAEs correspond to atypical auditory performance. If so, these two special populations will have been identified as interesting groups for study by other auditory specialists in order to understand the underlying mechanisms involved. In other experiments, OAEs, AEPs, and tympanometry will be measured in pre- and post-pubertal children. The results will be informative about the origin of the individual and sex differences seen in these measures for adults, and thus will be relevant to normal and abnormal development in the auditory system.